1. Field of the Invention
The present invention relates to a plasmon antenna for generating near-field light by being irradiated with light. And the present invention relates to a head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with near-field light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording density of a magnetic disk apparatus becomes higher, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic microparticles gathered together, and each of the magnetic microparticles has a single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic microparticles. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic microparticles and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume. Recently, as a method for solving this problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed, in which the magnetic recording medium is formed of a magnetic material with a large magnetic anisotropy energy KU, and writing is performed by reducing the anisotropic magnetic field with heat supplied to the magnetic recording medium just before applying write field.
As a thermally-assisted magnetic recording technique, a method has been generally known, in which a near-field light probe formed of a metal piece, so-called a plasmon antenna, is used for generating near-field light from plasmon that is excited by irradiated laser light. For example, U.S. Pat. No. 6,768,556 B1 discloses a plasmon antenna that includes a metal scatterer with a strobilus shape formed on a substrate and a dielectric material film formed around the metal scatterer. And US Patent Publication No. 2004/081031 A1 discloses a configuration in which a plasmon antenna is formed in contact with the main magnetic pole of a magnetic head for perpendicular magnetic recording in such a way that the irradiated surface of the plasmon antenna is perpendicular to the surface of a magnetic recording medium. Further, US Patent Publication No. 2003/066944 A1 discloses a technique in which the tip of a plasmon antenna is made closer to a magnetic recording medium to attempt to irradiate the medium with stronger near-field light.
As described above, various thermally-assisted magnetic recording techniques have been proposed, which uses plasmon antennas. The present inventors have devised a near-field light generating element in which laser light propagating through a waveguide (waveguide light) is coupled with a plasmon antenna in a surface plasmon mode to cause the excited surface plasmon to propagate to the opposed-to-medium surface, thereby providing near-field light, rather than directly applying the waveguide light to a plasmon antenna. The plasmon antenna in the element will be hereinafter referred to as a surface plasmon antenna. In the near-field light generating element, the temperature of the surface plasmon antenna does not excessively rise because waveguide light is not directly applied to the surface plasmon antenna. As a result, there can be avoided such a situation in which the end, which reaches the opposed-to-medium surface, of a read head element for reading data signal or servo signal from the magnetic recording medium becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the plasmon antenna, which makes it difficult to properly read the servo signal. In addition, there can also be avoided such a situation in which the light use efficiency of the near-field light generating element is degraded because thermal disturbance of free electrons increases in the plasmon antenna. Here, the light use efficiency of a near-field light generating element is given by IOUT/IIN(×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of near-field light emitted from a near-field light generating end of the plasmon antenna after converting the laser light into surface plasmon in the plasmon antenna.
The above-described near-field light generating element is required to have more improved light use efficiency. Actually, it is known that the light use efficiency of at least 10% (percent) or more is needed in order to achieve favorable thermally-assisted magnetic recording. To meet the requirement in the above-described near-field light generating element, especially, it is significantly important to couple as much amount as possible of the waveguide light with the plasmon antenna in a surface plasmon mode.